envkernel: Fix switching envkernel to different source path (!1733)

When there is already another kernel source path mounted then the old mount
should be removed. Otherwise the mount source will still be pointed at the
old kernel source.

helpers/envkernel.sh

@@ -128,7 +128,9 @@ initialize_chroot() {
 
 
 mount_kernel_source() {
-	[ -e "$chroot/mnt/linux/Kbuild" ] && return
+	if [ -e "$chroot/mnt/linux/Kbuild" ]; then
+		sudo umount "$chroot/mnt/linux"
+	fi
 	sudo mount --bind "$PWD" "$chroot/mnt/linux"
 }